The rock mole is a mole that eats through rock (surprise!). They are able to eat anything metal, including amulets and some rings, as well as zorkmids. Sometimes they will even eat the iron spikes out of pits! Surprisingly, though, they cannot actually eat rocks or stones, as no monster (currently) can.

A rock mole is a member of the rodent family. They get their
name from their ability to tunnel through rock in the same
fashion that a mole tunnels through earth. They are known to
eat anything they come across in their diggings, although it
is still unknown how they convert some of these things into
something of nutritional value.